Defibrillators are used to treat Sudden Cardiac Arrest with electrodes placed on the chest of the patient, and apply defibrillating shock to the heart of a cardiac arrest patient. The ECG of a cardiac arrest patient, properly measured and analyzed, indicates whether the patient's heart is exhibiting a shockable rhythm or a non-shockable rhythm. A shockable rhythm refers to an aberrant ECG which is subject to defibrillation, and restoration of a normal heartbeat, while a non-shockable rhythm refers to a normal ECG which needs no defibrillation, and to an aberrant ECG which is not subject to defibrillation. Ventricular fibrillation, for example, is a shockable rhythm, while pulseless electrical activity is an example of a non-shockable rhythm. Defibrillators are also capable of treating other dysrhythmias (irregular heartbeats), such as atrial fibrillation, bradycardia and tachycardia. The ECG of a patient can also indicate these conditions, and the defibrillator can be operated to apply cardioverting shock to the heart of patient experiencing dysrhythmias.
ECG diagnosis and delivery of shock can be accomplished with a ECG and a manual defibrillator, in which case a rescuer using the defibrillator analyzes the ECG to determine if the patient's ECG is shockable, and if so, apply the appropriate defibrillating shock to the patient. ECG diagnosis and delivery of shock can be accomplished with an Automatic External Defibrillator (AED), which is operable to automatically analyze the ECG to determine if the patient's ECG is shockable, and, if so, automatically apply the appropriate defibrillating shock to the patient (or advise an operator to operate the AED to apply the shock, in which case the devise may be referred to as a Semi-Automatic Defibrillator (SAD)). In each, the ECG is obtained through electrodes placed on the chest of the patient, and the defibrillating or cardioverting shock is applied through the same electrodes. For diagnosis and treatment of sudden cardiac arrest, the defibrillator uses two electrodes, with an apex electrode placed on the lower left front surface of the patient's chest, and the sternum electrode placed on the upper right front surface of the patient's chest. In diagnostic ECG monitoring systems, which are used to diagnose heart conditions in a patient not suffering from sudden cardiac arrest, many electrodes may be used to sense the ECG, and these electrodes are not useful for applying defibrillating or cardioverting shock.
The electrodes used with defibrillators are placed on the patient's body in clearly defined locations. A common arrangement for AED's is the anterior-apex scheme, described above, with an apex electrode disposed on the lower left front/medial surface of the patient's chest, just below and to the left of the pectoral muscle, and the sternum electrode (also referred to as the anterior electrode) placed on the upper right front surface of the patient's chest, below the right clavicle. For pacing, the preferred arrangement is the anterior-posterior scheme, which uses an anterior electrode disposed on the front left side of the chest, over the left precordium (that is, over the heart), and a posterior electrode disposed on the back of the patient, on the left side, beneath the heart and between the scapula and the spine at heart level. Diagnostic ECG systems use several small electrodes (up to fifteen electrodes in some systems) and electrode placement for these multi-lead systems is more complex.
Typical defibrillators use pre-packaged, multi-function, self-adhesive defibrillation (SAD) electrodes that both provide the functions of sensing the microvolt level activity of the heart's ECG signal as well as provide the conductive interface to the skin for delivering the therapeutic electrical shock. These self-adhesive defibrillation electrodes are typically much larger than standard commercial ECG electrodes used for diagnostic ECG: diagnostic ECG electrodes might have an active surface area of several square centimeters while self-adhesive defibrillation electrodes have a surface area of approximately 100 cm2. Defibrillation electrodes are also capable of handling current levels that are at least several orders of magnitude greater than diagnostic ECG electrodes.
In diagnosing and treating a sudden cardiac arrest patient, proper placement of the electrodes is critical to obtaining a correct ECG from the patient which can be used to determine whether the ECG indicates a shockable rhythm. In diagnosing a patient with chronic heart condition, proper placement of the electrodes is critical to obtaining a correct ECG from the patient which can be used to diagnose various heart conditions. Improper placement causes changes in the ECG obtained through the electrodes. These changes can result in an ECG trace with artifacts such that the trace cannot be analyzed, does not reveal a shockable rhythm, or incorrectly indicates a shockable rhythm. When attempting to diagnose a cardiac arrest patient, an unusable ECG may result in failure to treat the patient, or delay in treating the patient, and loss of the opportunity to revive the patient. In the diagnostic ECG, misplaced leads can lead to artifacts which simulate clinical pathology (ectopic atrial rhythm, ischemia, or infarction) that does not actually exist in the patient, and this can lead to unnecessary treatments and even unnecessary invasive procedures. The artifacts may or may not be detectable even by clinical experts. Also, proper placement of the electrodes is important to delivering the proper amount of defibrillating or cardioverting energy to a patient. Thus, in each case, a technician must carefully place the electrodes in predetermined locations on the patient's body.
Although ECG electrodes provided for use with AED's are often packaged with very clear directions for placement, electrode misplacement is still an occasional problem with serious, perhaps fatal consequences.